Method for preparing a composite printing form

ABSTRACT

The invention pertains to a method for preparing a composite printing form from a single precursor that is capable of forming a relief and a carrier. The single precursor can be a single photosensitive element or a single laser-engravable print element having a reinforced elastomeric layer. The single precursor has a size that is at least 70% of a size of the carrier. The single precursor is located on the carrier by approximately positioning the precursor on the carrier that has no registration markings. Precise registration of the single precursor is achieved by using digital information generated from a computer to create the registered image on the composite form. The method is particularly suited for preparing composite printing forms for relief printing, and in particular for preparing composite printing forms for flexographic printing of corrugated substrates.

BACKGROUND OF THE INVENTION

1. Field of the Disclosure

This invention pertains to a method for the preparation of compositeprinting forms, and particularly for the preparation of compositeprinting forms for use in relief printing.

2. Description of Related Art

Flexographic printing plates are widely used for printing of packagingmaterials including corrugated carton boxes, cardboard boxes, continuousweb of paper, and continuous web of plastic films. Flexographic printingplates are a form of relief printing in which ink is carried from araised-image surface and transferred to a substrate. Flexographicprinting plates can be prepared from photopolymerizable compositionsthat typically include an elastomeric binder, at least one monomer, anda photoinitiator, such as those described in U.S. Pat. Nos. 4,323,637and 4,427,759. The photosensitive elements generally have a solid layerof the photopolymerizable composition interposed between a support and acoversheet or a multilayer cover element.

Corrugated boxes and other, relatively large objects that are printedusing relief image printing plates often bear actual printing on only asmall portion of their total surface area. Those skilled in the artoften print relatively large objects with composite printing plates thatare prepared by mounting a plurality of relief image printing plates ona common carrier sheet. The individual plates, however, are mounted onlyon those portions of the carrier that correspond to the portions of theobject that actually need to be printed. However, this system formounting constituent relief image plates is laborious and requirescareful adhesion of the plates to the carrier while assuring accurateregistration on press for high quality printing and multi-colorreproduction. For multi-color reproduction, wherein a single plate isused for printing each of the individual colors, accurate registrationof the plates with respect to one another is particularly crucial.

Other methods of preparing composite printing plates have beensuggested, such as in U.S. Pat. No. 5,846,691; U.S. Pat. 6,312,871; U.S.Pat. No. 6,312,872; U.S. Pat. No. 6,399,281; and U.S. Pat. No.6,472,121, which do not require precise registration of constituentphotocurable elements. Generally the methods involve disposing at leastone photocurable element upon a surface of a carrier in approximateregister and then transferring a computer generated negative to asurface of the elements. The methods include transferring registrationinformation of any visually perceptible modification of the printingelement, i.e., carrier, that is intended to reflect positioning of thephotocurable elements. The negative or mask may be generated by jettingink onto the surface of the element or by exposing with laser radiationto selectively remove a radiation opaque layer from the surface of theelement.

Another way to print corrugated board and other large objects is toprepare a single relief image plate having a surface area correspondingto the total surface area of the object. Since only portion of theobject's surface needs to be printed, however, only a small portion ofthe relief image plate will actually be used for ink transfer. Theremainder of the plate will be unused, and is removed by washoutprocessing at the same time as the portion/s of the relief image isbeing formed. However, removal of the unused portion places extremestress on the washout apparatus and on the relief image plate. As such,large unused portions, i.e., unimaged portions, are cut away prior totreating.

In some end-use applications of corrugated board printing, onlyrelatively small areas of the corrugated substrate need to be printedcompared to the overall size of the component being formed from thecorrugated substrate. As such it is desirable to selectively mount aphotosensitive element on a carrier only at the location/s required forprinting.

It is desirable to assure the accurate positioning of the reliefprinting form or forms when mounting onto the print cylinder or onto thecarrier in order for the printed image/s on the substrate to beregistered. The relief printing form should be positioned on thecylinder such that the printing is parallel to the axis of the printingcylinder and not skewed. In multicolor printing, the relief printingform for each color being printed should be aligned so that thedifferent color printed images are registered with each other.Registration errors give rise to superimposed colors, spaces with nocolor, color shifts, and/or degraded image detail.

Even with these prior methods, there remains a need in the art foralternative processes for preparing composite printing forms. Inparticular, there remains a need for alternative, easy to use, processfor accurate registration of the constituent relief image printingplates on the composite printing form.

SUMMARY OF THE INVENTION

The present invention provides a method for preparing a compositeprinting form that includes a) providing one photosensitive elementcomprising a layer of a photopolymerizable composition and an infraredsensitive layer disposed above the photopolymerizable layer, the elementhaving a planar area; b) positioning the element adjacent a carrierhaving no marking or indicator for positioning of the element, whereinthe carrier has a planar area, and the planar area of the onephotosensitive element is at least 70% of the planar area of thecarrier; c) securing the element to the carrier; d) imagewise exposingthe infrared sensitive layer with infrared laser radiation to form amask on the element; e) overall exposing the element to actinicradiation through the mask; and f) treating the element of e) to form arelief structure on the carrier.

In accordance with another aspect of this invention there is provided amethod for preparing a composite printing form that includes a)providing one laser-engravable precursor comprising a reinforcedelastomeric layer, the precursor having a planar area; b) positioningthe precursor adjacent a carrier having no marking or indicator forpositioning of the precursor, wherein the carrier has a planar area, andthe planar area of the one precursor is at least 70% of the planar areaof the carrier; c) securing the precursor to the carrier; and d)exposing the reinforced layer with laser radiation to selectively removethe reinforced layer in depth and form a relief structure on thecarrier.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention is a method for preparing a composite printingform from a single precursor capable of forming a relief and a carrier.The single precursor can be a photosensitive element or alaser-engravable element having a reinforced elastomeric layer. Themethod is particularly suited for preparing composite printing forms forrelief printing, and in particular for preparing composite printingforms for flexographic printing of corrugated substrates. A compositeprint element is prepared by mounting one precursor on a face of thecarrier by visually estimating an approximate location for the precursoron the carrier, and digitally imaging the precursor to register itsprint image to a printing cylinder. The single precursor has a sizerelative to a size of the carrier, such that positioning of theprecursor at the approximate location assures that digital imaging cancreate on the precursor the image that will be printed by the compositeprint form.

The present invention provides a method that eliminates the tediousrequirement of having to hand-register individual relief printingelements in preparing composite printing forms. In some end-useapplications of corrugated board printing, a relatively large portion ofthe corrugated substrate is printed compared to the overall size of thecomponent being formed from the corrugated substrate. However, incorrugated board printing, the printing cylinder is not typicallyremovable from the printing press which makes it difficult to registerthe image of the printing plate to the printing cylinder. As such, aprint plate is not mounted directly to the corrugated board printingcylinder, and instead is mounted to a carrier which provides thecapability to register the image on the printing plate to the corrugatedboard printing cylinder and the corrugated board substrate.

According to the inventive method, one precursor of a suitable sizerequired is mounted on a carrier in approximate position, but not inregister, to areas that will ultimately print a substrate. Preciseregistration of the one precursor then is achieved by using digitalinformation generated from a computer to create the registered image onthe composite form. In some embodiments, precise registration of the oneprecursor is achieved by the precursor being a photosensitive elementand formation of a mask image using digital information generated from acomputer to the composite form. In other embodiments, preciseregistration of the one precursor is achieved by the precursor being alaser-engravable element having a reinforced elastomeric layer and usingdigital information generated from a computer to engrave, i.e., removematerial in depth from the layer, and form a relief structure for thecomposite form. This also avoids the complexity of prior methods thatinclude registration markings in the relief image of each of thephotosensitive elements for alignment with markings on the carrier.Also, the present invention can easily be integrated in usual productionprocesses for composite printing forms. No additional apparatus isnecessary to accomplish the present method. The present method ofpreparing a composite printing form by approximately positioning asingle print element on the carrier provides additional advantages,particularly compared to prior methods which directly mark the carrierto locate the photosensitive element. The direct mounting ofsuitably-sized precursor to the carrier can conserve on materialsconsumed, and reduce the cost of materials and/or the time to preparecomposite printing forms. The carrier can be re-used to preparesubsequent printing forms since the carrier will not having any markingsto confuse the positioning of the subsequent precursor or precursors.

In one embodiment, the single precursor that is used for preparingcomposite printing forms for relief printing is a photosensitive elementthat includes at least one photopolymerizable layer. In mostembodiments, the photosensitive element that is used for preparingcomposite printing forms for relief printing includes a support and theat least one photopolymerizable layer adjacent the support. Thephotopolymerizable layer is an elastomeric layer that includes a binder,at least one monomer, and a photoinitiator. Unlike some prior artmethods for mounting plates on carriers in which the relief image of theplate is already formed prior to mounting on the carrier, in mostembodiments the present method secures one photosensitive element thatis uncured or “raw”, i.e., has not been imagewise exposed, onto thecarrier. In one embodiment, the photosensitive element includes aninfrared sensitive layer disposed above the photopolymerizable layer(opposite the support) that is capable of forming a mask image basedupon digital information. The infrared sensitive layer can be integralwith the photosensitive element or can be associated with a separateelement which combines to form an assemblage. The uncured photosensitiveelement having the infrared sensitive layer may sometimes be referred toherein as a digital photosensitive element. In some other embodiments,the present method secures onto the carrier one precursor that is alaser-engravable element having a reinforced elastomeric layer. In somecases the laser-engravable element is a photosensitive element that isphotochemically reinforced, i.e., cured by overall or blanket exposureto actinic radiation, but the cured photosensitive element does notinclude unexposed portions and exposed portions or a relief image. Inembodiments in which the laser-engravable element is photochemicallyreinforced, the element having a reinforced elastomeric layer may alsobe referred to herein as a digital photosensitive element. Thephotosensitive element and the laser-engravable element may also bereferred to as a digital precursor.

The present method prepares the composite printing form by providing asingle precursor of a size sufficient to cover a majority of thecarrier. The one (i.e., single) precursor is suitably sized for mountingon the carrier, such that no template or registration mark/s on thecarrier is necessary to position the precursor to even an approximatelocation in order for the composite printing form to ultimately print inregister on the substrate. The one precursor has side edges withdimensions of a length and a width that determines a size or planar areaof the precursor, wherein the length and the width are each greater thanits thickness, and has two substantially flat opposing major faces.

The single precursor that is secured to the carrier forms a reliefstructure on the carrier that is suitable for printing by the compositeprinting form. The digital precursor on the carrier is converted to thecomposite printing form, i.e., relief printing structure on the carrier,by conducting a digital method to form an integrated image on theprecursor which assures precise registration. In some instances theintegrated image formed on the precursor is a mask image associated withthe photosensitive element, and then the photosensitive element on thecarrier is exposed to actinic radiation through the integrated mask, andtreated to remove the unexposed (i.e., uncured) portions to form therelief. In some other instances the integrated image formed on theprecursor is an engraved image forming a relief image, such that thecomposite printing form need not undergo any further steps to form therelief suitable for printing. Digital methods relate to any method ofdigitally (i.e., computer-controlled) transferring of graphicinformation, including text and images, generated from a computer (datafile(s)) to the precursor on the composite printing form. In someembodiments, digital methods involve the formation of an image disposedabove the photosensitive layer and integrated with the photosensitiveelement so as to serve as an integrated mask image of opaque areas andclear areas. In other embodiments, digital methods involve directlyforming the relief image in the elastomeric layer. Digital methods mayalso be referred to as computer-to-plate technology or methods. Digitalmethods may also be referred to herein as digital technology, or digitalimage technology.

As is well known to those skilled in the art, the design of graphicinformation, including text and images, that is to be printed by thecomposite printing form is captured and manipulated in computer softwareto create data files including page layout data. The page layout datacan be based upon vector or bitmapped data representing one or more ofthe locations that graphic information will be printed on the “page” orsubstrate. The page layout data can be used to determine the number andsizes of the photosensitive elements that are needed for a compositeprinting form. Based on the page layout data, a computer typicallygenerates a cutting guide for the digital precursor. However, thecomputer or an operator may determine that cutting the precursor intotwo or more segments is not desirable or necessary, and elect to use asingle precursor on the carrier. The image may have a size and/or thegraphic information may be of such a nature, that it would beinefficient use of material and/or resources, to cut the precursor intotwo or more segments that need to be separately mounted. The compositeprinting form of the present invention includes only one digitalprecursor on the carrier. Even so, it should be noted that a precursormay be cut to a suitable size relative to a size of the carrier thatwill be used. The precursor is placed on an automatic cutting table thatis connected to the computer and can be cut into the suitable size basedon the page layout data. A particularly suitable cutting table is aKongsberg digital converting table, manufactured by EskoArtwork. Inanother embodiment, the precursor can be cut manually into the suitablesize based on the page layout data. In the present invention, thecomposite printing form includes only one precursor having a suitablesize, i.e., planar area, relative to the size, i.e., planar area, of thecarrier.

For the present composite printing form, the one precursor has a sizethat is at least in accordance with a size of the printed precursorelement is suitably sized when the planar area of the precursor is atleast 70% of the planar area of the carrier. That is, at least 70% ofthe planar area of the carrier is covered by the single digitalprecursor when secured on the carrier. The planar area of the singleprecursor expressed as a percentage of the planar area of the carriercan occur in any percentage increment, including portions of anincrement, from about 70% to about 99%. In some embodiments, the planararea of the one precursor is less than about 99% of the planar area ofthe carrier. In some embodiments, the planar area of the one precursoris less than 98% of the planar area of the carrier. In some embodiments,the planar area of the one precursor is less than 96% of the planar areaof the carrier. In other embodiments, the planar area of the oneprecursor is 75% of the planar area of the carrier. In yet otherembodiments, the planar area of the one precursor can be from about 70%to 98% of the planar area of the carrier. In still other embodiments,the planar area of the one precursor can be about 70% to 96% of theplanar area of the carrier. The single precursor having its planar areathat is 70% to about 99% of the planar area of the carrier assures thatthe digital precursor can be approximately positioned on the carrierwithout the need for registration marks or other indicators on thecarrier in order for the image on the page layout to be fully created onthe precursor by digital methods.

In most embodiments, the single precursor does not completely cover thecarrier, that is, the planar area of the precursor is less than 100% ofthe planar area of the carrier. The following should be considered by anoperator or preparer of the composite printing form when positioning theprecursor adjacent the carrier. The carrier requires at least sufficientopen space that is not covered by the precursor to allow for theattachment of the mounting bar at its leading edge. In some embodiments,about 1 to 3 inch (2.54 to 7.6 cm) of open space is allotted at theleading edge of the carrier to accommodate the attachment of themounting bar. In most embodiments, the planar area of the precursor issuch that at least the length of the precursor is less the length of thecarrier. In some embodiments, both the length and width dimensions ofthe precursor are less than the length and width dimensions of thecarrier, which provides a margin of open space on the carrier in whichthe precursor does not reside. The margin of open space on the carrierrelative to the precursor may be substantially equally spaced about theside edges of the element or may not be equally spaced about the sideedges of the element. In many embodiments, the precursor should not besuperposed on the carrier to align or abut one or more of the side edgesof the precursor to coincide with the corresponding side edges of thecarrier. Particularly in embodiments when the precursor is aphotosensitive element, some open space about the precursor on thecarrier is needed to provide for the application of an edge coveringmaterial on the precursor. In many embodiments the margin of open spaceon the carrier essentially surrounds the precursor. The margin of openspace surrounding the precursor need only be sufficiently large enoughto allow for the application of an edge covering material to a lateralarea of the side edges of the precursor. The edge covering material isapplied to the lateral edges of the precursor after the element ismounted to the carrier, and is used to block exposure by actinicradiation at the lateral edges.

Positioning the digital precursor adjacent the carrier places theelement at an approximate location on the carrier that will encompassthe printed image based on the page layout data. Since the carrieritself is not modified to carry or include any registration information,the carrier has a planar surface that excludes the presence of markingsor other registration information. In most embodiments, the precursor ispositioned by visual estimation of an approximate location on thecarrier, taking into account the need for open space at the leading edgeof the carrier for the mounting bar, and for open space about the sideedges of the precursor. The precursor does not overhang one or more sideedges of the carrier. Since the planar surface area of the precursor isabout 70% to about 99% of the planar area of the carrier, the placementof the element at the approximate location typically assures that theentire image that is to be printed will be captured on the digitalprecursor during imagewise exposure. The printed image will beregistered to the carrier by the imagewise exposure of a digital method.In most embodiments, the precursor is positioned by visually estimatingthe approximate location where the photosensitive element issubstantially centered or offset from center on the carrier.

The carrier holds and maintains the position of the precursor forprinting. In most embodiments, the carrier is in sheet form, but is notso limited, and can include cylindrically-shaped forms. The carrier, insheet form, has side edges with dimensions of a length and a width thatdetermines a size or planar area of the carrier, wherein the length andthe width are substantially greater than its thickness, and has twosubstantially planar opposing major faces. In embodiments where thecarrier is cylindrically-shaped, the size is based on a surface area ofan exterior surface of the cylindrically-shaped carrier that secures theone precursor (excludes end surfaces of the cylinder), wherein a lengthis comparable to a perimeter of the exterior surface of the cylindricalcarrier and a width is comparable to a height of the cylindricalcarrier. The carrier has no marking/s or indicator/s for positioning ofthe single photosensitive element. Since in the present invention thecarrier sheet is not marked, in some embodiments the carrier of a firstcomposite printing form can be recycled, or re-used in a secondcomposite printing form after the one precursor is removed from thecarrier of the first composite printing form. (If the carrier isre-used, irreversibly marking the carrier with registration indicatorscan create confusion on the placement of the segment/s of precursor forthe second composite printing form.)

Materials suitable for use as the sheet carrier can be any which areflexible to wrap about the print cylinder, resistant to deformation andstretching particularly during printing, and resistant to deformation orsize change upon treating. Examples of materials suitable as the carrierinclude, but are not limited to, vinyl plastics, linoleum, metal, andpolymeric films, such as polyester. Suitable thickness of the carrier isform 0.005 to 0.185 inch (0.013 to 0.47 cm). In many embodiments, thecarrier is a polymeric film of polyvinylchloride (PVC) or polyethyleneterephthalate that has a thickness from 0.010 to 0.030 inch (0.025 to0.076 cm). Materials suitable for use as the cylindrically-shapedcarrier can be any supports that are for use in cylindrical printingforms, commonly referred to as print sleeves or sleeves. The type ofsleeve or cylindrical support is not limited by the present invention.The sleeve may be formed from single layer or multiple layers offlexible material. In some embodiments, flexible sleeves made ofpolymeric films are suitable. In other embodiments, sleeves of metal,such as nickel; or glass epoxy, are suitable. The sleeve can have a wallthickness from less than 10 mils (0.025 cm) to 80 mils (0.203 cm) ormore. It is also contemplated that opposing ends of the carrier sheetcan be joined by any appropriate means, such as for example, meltfusing, taping, stitching, clamping, stapling, taping, gluing, andsewing, to form a cylindrically-shaped carrier. The carrier can beformed into a cylindrical shape prior to, but preferably after, theprecursor is secured to the carrier sheet.

The carrier (in sheet form) has a leading end or leading edge where atleast two openings or holes are located. The at least two holes arespaced apart in a row located near and parallel to the leading end. Thesize, the shape, and number of the holes (beyond the at least two holes)are not particularly limited for the purposes of the present invention.The at least two holes in the leading ends of the carrier provide thecapability to register the carrier to the printing cylinder (and in thelaser exposure apparatus). The at least two holes in the leading ends ofthe carrier can also be used to secure the carrier to a mounting bar formounting. The holes in the leading end of the carrier can be machined,punched, or drilled by conventional equipment. Sources of conventionalpunching equipment are manufacturers of film punches for the printingindustry such as Stoesser, Burgess Industries, and Carlson. Afterpunching holes in each of the leading ends of the carrier, the singlephotosensitive element is positioned adjacent the carrier. In mostembodiments, the photosensitive element is positioned on top of thecarrier.

In embodiments where the carrier is cylindrically-shaped, the carriermay be (pre) mounted onto a support cylinder associated with an imagingor other apparatus and the one photosensitive element is positionedadjacent the cylindrical carrier. The photosensitive element can be heldin position on the carrier while mounting by taping or pinning one ormore ends of the photosensitive element to the carrier.

The single digital precursor is mounted to the carrier and residessecurely on the carrier in a position that will allow for registrationof the image by a digital method. Thus, the one digital precursor ismounted on the carrier at the approximate location by the visualestimation of a preparer of the composite printing form. In oneembodiment, the photosensitive element as the precursor is mounted ontothe carrier such that the side of the element opposite the infraredsensitive layer resides on or is adjacent the carrier. In mostembodiments, the support of the photosensitive element will be adjacentthe carrier. The one precursor is positioned and mounted on the carriersuch that the planar area of the element covers about 70% to about 99%of the planar area of the carrier. Accurate registration is achieved forthe composite printing form by the computer-controlled digital transferof graphic information to an exterior surface of the precursor (oppositethe support) that has been mounted on the carrier.

The one precursor can be secured or mounted using any of the many meansknown to those skilled in the art. Most embodiments involve applyingdouble-sided adhesive tape or some other suitable adhesive or glue tothe photosensitive element, to the carrier, or to both. Other methods ofsecuring the precursor to the carrier are encompassed within the presentinvention and include, but are not limited to, pinning, stitching, andstapling. In some embodiments, the double-sided adhesive tape is appliedto the support of the precursor. In other embodiments, the support isremoved from the precursor and the double-sided adhesive tape is appliedto a surface of the precursor where the support was previously.

In the embodiment in which the precursor is a photosensitive element,the photosensitive element includes an infrared sensitive layer disposedabove the photopolymerizable layer, the integrated mask is formeddigitally from the infrared sensitive layer, and then the photosensitiveelement is exposed to actinic radiation through the integrated mask, andtreated to remove the unexposed portions of the layer and form therelief suitable for printing. In preparation for exposing the onephotosensitive element to actinic radiation through a mask, an in-situmask image is first formed on or disposed above the surface of thephotopolymerizable layer opposite the support. The in-situ mask may alsobe referred to as an integrated mask image, or integrated mask. TheIR-sensitive layer can itself form an integrated masking layer for thephotosensitive element or can be used in conjunction with one or moreadjacent (radiation opaque) layers to form the integrated mask layer onthe element. The mask includes opaque areas and “clear” areas that formthe image. The opaque areas of the mask prevent the photopolymerizablematerial beneath from being exposed to the radiation and hence thoseareas of the photopolymerizable layer covered by the dark areas do notpolymerize. The “clear” areas of the mask expose the photopolymerizablelayer to actinic radiation and polymerize or crosslink. The imagenecessary for the imagewise exposure of the photopolymerizable layer canbe generated by any digital method. Precise registration of the reliefimages in the photosensitive element on the carrier is achieved throughthe formation of the integrated mask image on the element by digitaltechniques conducted after the element is approximately located andsecured to the carrier.

The digital photosensitive element includes an infrared sensitive layer,which in some embodiments can also function as an actinic radiationopaque layer, that is used in digital image technology in which laserradiation is used to form a mask for the photosensitive element (insteadof the conventional phototool film image). Digital methods create a maskimage in situ on or disposed above the photopolymerizable layer withlaser radiation. Digital methods of creating the mask image require oneor more steps to prepare the photosensitive element prior to imagewiseexposure. Generally, digital methods of in situ mask formation eitherselectively remove or transfer the radiation opaque layer, from or to asurface of the photosensitive element opposite the support. The presenceof materials in the infrared sensitive layer that are black, such asdark inorganic pigments, such as carbon black and graphite, mixtures ofpigments, metals, and metal alloys function as both infrared-sensitivematerial and radiation-opaque material. The infrared laser exposure canbe carried out using various types of infrared lasers, which emit in therange 750 to 20,000 nm. Infrared lasers including, diode lasers emittingin the range 780 to 2,000 nm and Nd:YAG lasers emitting at 1064 nm arepreferred. The in situ mask images remain on the photosensitive elementfor subsequent steps of overall exposure to actinic radiation.

One suitable infrared laser exposure apparatus used for digitalformation an in-situ mask on a surface of the photosensitive element isa CYREL® Digital Imager (sold by EskoArtwork, from Gent, Belgium).Conventionally, the laser exposure apparatus includes a rotating drumhaving a clamp that holds one end of a planar photosensitive printingelement, and securely mounts the element to the drum during laserimaging. The drum may include a second clamp for holding an opposite endof the planar element. The drum and/or the clamp of a laser exposureapparatus may be modified to accommodate mounting of the compositeprinting form. In one embodiment, a removable mounting bar can be usedfor mounting the composite printing form in the laser exposureapparatus. The removable mounting bar has a length that is the same orsubstantially the same as a length of the clamp on the drum (i.e.,width). Along its length, the mounting bar includes one or more openingsto align to the drum and one or more raised pins to align the compositeprint form to the mounting bar (and drum). The drum may have one or morepins at or adjacent the clamp that mate with the one or more openings inthe mounting bar. The mating of the drum pin/s with the opening/s in themounting bar registers the mounting bar to the drum (and laser). Theraised pins on the mounting bar are positioned to correspond to theholes at the leading end of the carrier. The composite print form issecured to the drum by engaging the holes at the leading end of thecarrier with the raised pins on the mounting bar and capturing theleading end of the carrier with the clamp. Along its length, portions ofthe clamp may be cutaway to accommodate the raised pins on the mountingbar when the mounting bar and the composite printing form are secured bythe clamp. In some embodiments, the mounting bar for the drum isseparate and distinct from a mounting bar used to secure the compositeprint form to a print cylinder. In other embodiments, the drum and clampof the laser exposure apparatus may be modified to accommodate amounting bar, which is the same as the mounting bar used to secure thecomposite print form to a print cylinder.

In one digital method, the photosensitive element will initially includethe infrared sensitive layer that covers or substantially covers theentire surface of the photopolymerizable layer. The infrared sensitivelayer is exposed imagewise to infrared laser radiation to form the imageon or disposed above the photopolymerizable layer, i.e., the in situmask. The infrared laser radiation can selectively remove, e.g., ablateor vaporize, the infrared sensitive layer (i.e., radiation opaque layer)from the photopolymerizable layer, as disclosed by Fan in U.S. Pat. Nos.5,262,275 and 5,719,009; and Fan in EP 0 741 330 B1. A material capturesheet adjacent the infrared sensitive layer may be present during laserexposure to capture the material as it is removed from thephotosensitive element as disclosed by Van Zoeren in U.S. Pat. No.5,506,086. Only the portions of the infrared sensitive layer that werenot removed from the photosensitive element will remain on the elementforming the in situ mask of radiation opaque material.

In another digital method of mask formation, the photosensitive elementwill not initially include the infrared sensitive layer. A separateelement bearing the infrared sensitive layer as a radiation opaque layerwill form an assemblage with the photosensitive element such that theradiation opaque layer is adjacent the surface of the photosensitiveelement opposite the support, which is typically the photopolymerizablelayer. (If present, a coversheet associated with the photopolymerizablelayer is removed prior to forming the assemblage). The separate elementmay include one or more other layers, such as ejection layers or heatinglayers, to aid in the digital exposure process. These other layers mayalso be considered an infrared sensitive layer. Hereto, the radiationopaque layer is also sensitive to infrared radiation. The assemblage isexposed imagewise with infrared laser radiation to selectively transferthe radiation opaque layer and form the image on or disposed above thephotopolymerizable layer as disclosed by Fan et al. in U.S. Pat. No.5,607,814; and Blanchett in U.S. Pat. Nos. 5,766,819; 5,840,463; and EP0 891 877 A. Only the portions of the radiation opaque layer which weretransferred will reside on the photosensitive element forming the insitu mask.

It is also contemplated that the mask image may be created on a separatecarrier and then transferred by application of heat and/or pressure tothe surface of the photopolymerizable layer opposite the support. Thephotopolymerizable layer is typically tacky and will retain thetransferred image. Optionally, the separate carrier can then be removedfrom the element prior to imagewise exposure. The separate carrier mayhave an infrared sensitive layer which is also a radiation opaque layeror associated with a radiation opaque layer, that is imagewise exposedto infrared laser radiation to selectively remove the radiation opaquematerial and form the image. An example of this type of separate carrieris LaserMask® imaging film by Rexam, Inc. Alternatively, the image ofradiation opaque material may be transferred to the separate carrierfrom another element having the radiation opaque material by laserradiation. The separate carrier may include at least two holes along itsleading edge for locating the separate carrier on the pin bar adjacentthe carrier and in alignment with the photosensitive element secured tothe carrier.

The imagewise exposure with infrared laser radiation can be carried outusing various types of infrared lasers, which emit in the range 750 to20,000 nm. Infrared lasers including diode lasers emitting in the range780 to 2,000 nm and Nd:YAG lasers emitting at 1064 nm are preferred. Apreferred apparatus and method for infrared laser exposure to imagewiseremove the actinic radiation opaque layer from the photosensitiveelement is disclosed by Fan et al. in U.S. Pat. Nos. 5,760,880 and5,654,125. The integrated mask image(s) remain on the photosensitiveelement for subsequent step of overall exposure to actinic radiation.

The next step of one embodiment of the process of the present inventionis to overall expose the photosensitive element to actinic radiationthrough the integrated mask image, that is, imagewise exposure of theelement through the digitally formed mask. The photosensitive element isexposed through the mask to actinic radiation from suitable sources. Theactinic radiation exposure time can vary from a few seconds to minutes,depending upon the intensity and spectral energy distribution of theradiation, its distance from the photosensitive element, the desiredimage resolution, and the nature and amount of the photopolymerizablecomposition. Exposure temperatures are preferably ambient or slightlyhigher, i.e., about 20° to about 35° C. Exposure is of sufficientduration to crosslink the exposed areas down to the support or to theback exposed layer, i.e., floor. Imagewise exposure time is typicallymuch longer than backflash exposure time, and ranges from a few to tensof minutes.

Actinic radiation sources encompass the ultraviolet and visiblewavelength regions. The suitability of a particular actinic radiationsource is governed by the photosensitivity of the initiator and the atleast one monomer used in preparing the photosensitive element. Thepreferred photosensitivity of most common photosensitive elements forrelief printing are in the UV and deep UV area of the spectrum, as theyafford better room-light stability. Examples of suitable visible and UVsources include carbon arcs, mercury-vapor arcs, fluorescent lamps,electron flash units, electron beam units, lasers, and photographicflood lamps. The most suitable sources of UV radiation are the mercuryvapor lamps, particularly the sun lamps. Examples of industry standardradiation sources include the Sylvania 350 Blacklight fluorescent lamp(FR48T12/350 VLNHO/180, 115w), and the Philips UV-A “TL”-serieslow-pressure mercury-vapor fluorescent lamps. Typically, a mercury vaporarc or a sunlamp can be used at a distance of about 1.5 to about 60inches (about 3.8 to about 153 cm) from the photosensitive element.These radiation sources generally emit long-wave UV radiation between310-400 nm. Flexographic printing plates sensitive to these particularUV sources use photoinitiators that absorb between 310-400 nm.

Imagewise exposure of the photosensitive element having the in-situ maskto actinic radiation is not limited, and may be conducted in thepresence of atmospheric oxygen, or in the absence of atmospheric oxygen,or in an atmosphere having a controlled amount of oxygen less thanatmospheric. Atmospheric oxygen is eliminated when the exposure isconducted in a vacuum. The exposure may be conducted in a vacuum tominimize the effects of oxygen on the polymerization reactions occurringin that layer. The exposure may be conducted in the presence ofatmospheric oxygen since the mask is formed in situ or applied imagewisewith radiation opaque material on the photopolymerizable layer, there isno need for vacuum to assure intimate contact of the in-situ mask. Insome embodiments of the process of preparing the composite printingform, the overall exposure step is conducted without vacuum, i.e., whilethe photosensitive element is in the presence of atmospheric oxygen, andwithout any additional layers present on top of the in-situ mask. Inother embodiments of the process of preparing the composite printingform, the overall exposure step is conducted in an atmosphere having aninert gas and a controlled amount of oxygen less than atmosphericoxygen. Suitable methods of overall exposing the digital photosensitiveelement in an environment having an inert gas and a controlled oxygenconcentration in a range between 190,000 parts per million (ppm) and 100ppm is described in U.S. Ser. No. 12/349608, filed Jan. 7, 2009(Attorney docket number IM-1346); and U.S. Ser. No. 12/401,106, filedMar. 10, 2009 (Attorney docket number IM-1359CIP).

The process of the invention usually includes a back exposure orbackflash step. This is a blanket exposure to actinic radiation throughthe support of the photosensitive element. It is used to create a layerof polymerized material, or a floor, on the support side of thephotopolymerizable layer and to sensitize the photopolymerizable layer.The floor provides improved adhesion between the photopolymerizablelayer and the support, helps highlight dot resolution and alsoestablishes the depth of the plate relief. The backflash exposure cantake place before, after or during the other imaging steps. Any of theconventional radiation sources discussed above for the overall(imagewise) actinic radiation exposure step can be used for thebackflash exposure step. Exposure time generally range from a fewseconds up to a few minutes. The back exposure step can be performedafter securing the one photosensitive element to the carrier providingthat the carrier and the mounting means are sufficiently transparent toactinic radiation, but in most embodiments the back exposure isperformed before the photosensitive element is secured to the carrier.

Following overall exposure to UV radiation through the mask, thephotosensitive element is treated to remove unpolymerized areas in thephotopolymerizable layer and thereby form a relief image. The treatingstep removes at least the photopolymerizable layer in the areas whichwere not exposed to actinic radiation, i.e., the unexposed areas oruncured areas, of the photopolymerizable layer. Except for theelastomeric capping layer, typically the additional layers that may bepresent on the photopolymerizable layer are removed or substantiallyremoved from the polymerized areas of the photopolymerizable layer. Forphotosensitive elements having the integrated mask image, the treatingstep also removes the mask image (which had been exposed to actinicradiation) and the underlying unexposed areas of the photopolymerizablelayer.

Treatment of the photosensitive printing element includes (1) “wet”development wherein the photopolymerizable layer is contacted with asuitable developer solution to washout unpolymerized areas and (2) “dry”development wherein the photosensitive element is heated to adevelopment temperature which causes the unpolymerized areas of thephotopolymerizable layer to melt or soften or flow and is wicked away bycontact with an absorbent material. Dry development may also be calledthermal development.

Wet development is usually carried out at about room temperature. Thedevelopers can be organic solvents, aqueous or semi-aqueous solutions,and water. The choice of the developer will depend primarily on thechemical nature of the photopolymerizable material to be removed.Suitable organic solvent developers include aromatic or aliphatichydrocarbon and aliphatic or aromatic halohydrocarbon solvents, ormixtures of such solvents with suitable alcohols. Other organic solventdevelopers have been disclosed in published German Application 38 28551. Suitable semi-aqueous developers usually contain water and a watermiscible organic solvent and an alkaline material. Suitable aqueousdevelopers usually contain water and an alkaline material. Othersuitable aqueous developer combinations are described in U.S. Pat. No.3,796,602.

Development time can vary, but it is preferably in the range of about 2to about 25 minutes. Developer can be applied in any convenient manner,including immersion, spraying and brush or roller application. Brushingaids can be used to remove the unpolymerized portions of the element.Washout can be carried out in an automatic processing unit which usesdeveloper and mechanical brushing action to remove the unexposedportions of the plate, leaving a relief constituting the exposed imageand the floor.

Following treatment by developing in solution, the relief printingplates are generally blotted or wiped dry, and then more fully dried ina forced air or infrared oven. Drying times and temperatures may varyhowever typically the plate is dried for 60 to 120 minutes at 60° C.High temperatures are not recommended because the support can shrink andthis can cause registration problems.

Treating the element thermally includes heating the photosensitiveelement having at least one photopolymerizable layer (and the additionallayer/s) to a temperature sufficient to cause the uncured portions ofthe photopolymerizable layer to soften or melt or flow, and contactingan outermost surface of the element to an absorbent surface to absorb orwick away the melt or flow portions. The polymerized areas of thephotopolymerizable layer have a higher melting temperature than theunpolymerized areas and therefore do not melt, soften, or flow at thethermal development temperatures. Thermal development of photosensitiveelements to form flexographic printing plates is described by Martens inU.S. Pat. Nos. 5,015,556; 5,175,072; 5,215,859; and by Wang et al. in WO98/13730.

The term “melt” is used to describe the behavior of the unirradiatedportions of the photopolymerizable elastomeric layer subjected to anelevated temperature that softens and reduces the viscosity to permitflow and absorption by the absorbent material. The material of themeltable portion of the photopolymerizable layer is usually aviscoelastic material which does not have a sharp transition between asolid and a liquid, so the process functions to absorb the heatedcomposition layer at any temperature above some threshold for absorptionin the absorbent material. A wide temperature range may be utilized to“melt” the composition layer for the purposes of this invention.Absorption may be slower at lower temperatures and faster at highertemperatures during successful operation of the process.

The thermal treating steps of heating the photosensitive element andcontacting an outermost surface of the element with an absorbentmaterial can be done at the same time, or in sequence provided that theuncured portions of the photopolymerizable layer are still soft or in amelt state when contacted with the absorbent material. The at least onephotopolymerizable layer (and the additional layer/s) are heated byconduction, convection, radiation, or other heating methods to atemperature sufficient to effect melting of the uncured portions but notso high as to effect distortion of the cured portions of the layer. Theone or more additional layers disposed above the photopolymerizablelayer may soften or melt or flow and be absorbed as well by theabsorbent material. The photosensitive element is heated to a surfacetemperature above about 40° C, preferably from about 40° C. to about230° C. (104-446° F.) in order to effect melting or flowing of theuncured portions of the photopolymerizable layer. By maintaining more orless intimate contact of the absorbent material with thephotopolymerizable layer that is molten in the uncured regions, atransfer of the uncured photosensitive material from thephotopolymerizable layer to the absorbent material takes place. Whilestill in the heated condition, the absorbent material is separated fromthe cured photopolymerizable layer in contact with the support layer toreveal the relief structure. A cycle of the steps of heating thephotopolymerizable layer and contacting the molten (portions) layer withan absorbent material can be repeated as many times as necessary toadequately remove the uncured material and create sufficient reliefdepth. However, it is desirable to minimize the number of cycles forsuitable system performance, and typically the photopolymerizableelement is thermally treated for 5 to 15 cycles. Intimate contact of theabsorbent material to the photopolymerizable layer (while in the uncuredportions are melt) may be maintained by the pressing the layer and theabsorbent material together.

A preferred apparatus to thermally develop the photosensitive element isdisclosed by Peterson et al. in U.S. Pat. No. 5,279,697, and also byJohnson et al. in U.S. Pat. No. 6,797,454. The composite printing formcarrying the one photosensitive element may be placed on a drum or aplanar surface in order for thermal treatment to be carried out.

The absorbent material is selected having a melt temperature exceedingthe melt temperature of the uncured portions of the photopolymerizablelayer and having good tear resistance at the same operatingtemperatures. Preferably, the selected material withstands thetemperatures required to process the photosensitive element duringheating. The absorbent material is selected from non-woven materials,paper stocks, fibrous woven material, open-celled foam materials, porousmaterials that contain more or less a substantial fraction of theirincluded volume as void volume. The absorbent material can be in web orsheet form. The absorbent materials should also possess a highabsorbency for the molten elastomeric composition. Preferred is anon-woven nylon web. It is also contemplated that the photosensitiveelement may undergo one or more treating steps to sufficiently removethe uncured portions to form the relief. The photosensitive element mayundergo both wet development and dry development, in any order, to formthe relief. A pre-development treating step may be necessary to removeone or more of the additional layers disposed above thephotopolymerizable layer if such additional layers are not removable bythe washout solution and/or by heating.

The photosensitive element can be uniformly post-exposed to actinicradiation to ensure that the photopolymerization process is complete andthat the element will remain stable during printing and storage. Thispost-exposure step can utilize the same radiation source as the mainoverall exposure.

Detackification is an optional post-development treatment that can beapplied if the surface of the photosensitive printing element is stilltacky, such tackiness not generally being removed in post-exposure.Tackiness can be eliminated by methods well known in the art, such astreatment with bromine or chlorine solutions. Preferably,detackification is accomplished by exposure to radiation sources havinga wavelength not longer than 300 nm, as disclosed in European PublishedPatent Application 0 017927 and Gibson U.S. Pat. No. 4,806,506.

As an alternate embodiment of the present method the one precursor isthe laser-engravable element having a reinforced elastomeric layer thatcan be engraved with laser radiation to form a relief surface suitablefor printing. The one laser-engravable element can be mounted on thecarrier before or after reinforcement of the elastomeric layer. The onelaser-engravable element is mounted to the carrier before selectivelyexposing to laser radiation to engrave the relief on the compositeprinting form. The one laser-engravable element on the carrier isconverted to the composite printing form by using digital informationfor engrave the relief image. In most embodiments, the onelaser-engravable element is a photosensitive element, i.e., the digitalphotosensitive element, on the carrier that is reinforced by overallexposing the layer of a photopolymerizable composition. In otherembodiments, the one laser-engravable element on the carrier isreinforced mechanically, or thermochemically. In yet other embodiments,the elastomeric layer of the laser-engravable element can be reinforcedby combinations of photochemical, mechanical, and/or thermochemicalreinforcement. The laser-engravable element may be suitably reinforcedand then exposed to laser radiation to engrave or selectively remove thereinforced layer in depth imagewise. U.S. Pat. No. 5,798,202; U.S. Pat.No. 5,804,353; and U.S. Pat. No. 6,757,216 B2 disclose suitableprocesses for making a flexographic printing plate by laser engraving areinforced elastomeric layer on a flexible support, which are herebyincorporated by reference. The processes disclosed in U.S. Pat. Nos.5,798,202 and 5,804,353 involve reinforcing and laser engraving asingle-layer, or one or more layers of a multi-layer, of a flexographicprinting element comprised of a reinforced elastomeric layer on aflexible support. Thermochemical reinforcement is accomplished byincorporating materials, which undergo hardening reactions when exposedto heat, into the elastomeric layer. Mechanical reinforcement isprovided by incorporating reinforcing agents, such as finely dividedparticulate material, into the elastomeric layer. Photochemicalreinforcement is accomplished by incorporating photohardenable materialsinto the elastomeric layer and exposing the layer to actinic radiation.Photohardenable materials include photocrosslinkable andphotopolymerizable systems having a photoinitiator or photoinitiatorsystem, and are encompassed by the photosensitive elements and methodsof use described herein. It is well within the skill of those in the artto appropriately select materials suitable for thermochemically,mechanically, and/or photochemically reinforcing the elastomeric layerof the laser-engravable element.

The term “laser engravable” as used herein refers to materials capableof absorbing laser radiation such that those areas of the materialswhich are exposed to a laser beam of sufficient intensity becomephysically detached with sufficient resolution and relief depth to besuitable for flexographic applications. By “physically detached”, it ismeant that the material so exposed is either removed or is capable ofbeing removed by any mechanical means such as by vacuum cleaning orwashing or by directing a stream of gas across the surface to remove theloosened particles.

The present method prepares the composite printing form by providing asingle laser-engravable element of a size sufficient to cover a majorityof the carrier. The one (i.e., single) laser-engravable element issuitably sized for mounting on the carrier, such that no template orregistration mark/s on the carrier is necessary to position the elementto even an approximate location in order for the composite printing formto ultimately print in register on the substrate. The laser-engravableelement on the carrier is converted to the composite printing form by adigital method that selectively exposes with laser radiation to engravethe reinforced elastomeric layer and directly form the relief image,which assures precise registration.

After securing the one laser-engravable element to the carrier, theelastomeric layer of the element is engraved with laser radiation. Laserengraving involves the absorption of laser radiation, localized heatingand removal of material in three dimensions. The laser engraving processof the invention does not involve the use of a mask or stencil. This isbecause the laser impinges the reinforced elastomeric layer to beengraved at or near its focus spot. Thus the smallest feature that canbe engraved is dictated by the laser beam itself. The laser beam and thematerial to be engraved are in constant motion with respect to eachother, so that each minute area of the plate (pixel) is individuallyaddressed by the laser. The image information is fed into this type ofsystem directly from the computer as digital data, rather than via astencil. Any pattern of a single image or multiple images of the same ordifferent images may be engraved.

Factors to be considered when laser engraving include, but are notlimited to, deposition of energy into the depth of the element, thermaldissipation, melting, vaporization, thermally-induced chemical reactionssuch as oxidation, presence of air-borne material over the surface ofthe element being engraved, and mechanical ejection of material from theelement being engraved. Investigative efforts with respect to engravingof metals and ceramic materials with a focused laser beam havedemonstrated that engraving efficiency (the volume of material removedper unit of laser energy) and precision are strongly intertwined withthe characteristics of the material to be engraved and the conditionsunder which laser engraving will occur. Similar complexities come intoplay when engraving elastomeric materials even though such materials arequite different from metals and ceramic materials.

Laser engravable materials usually exhibit some sort of intensitythreshold, below which no material will be removed. Below the thresholdit appears that laser energy deposited into the material is dissipatedbefore the vaporization temperature of the material is reached. Thisthreshold can be quite high for metals and ceramic materials. However,with respect to elastomeric materials it can be quite low. Above thisthreshold, the rate of energy input competes quite well with opposingenergy loss mechanisms such as thermal dissipation. The dissipatedenergy near, though not in, the illuminated area may be sufficient tovaporize to material and, thus, the engraved features become wider anddeeper. This effect is more pronounced with materials having low meltingtemperatures.

Laser engraving can be accomplished by any of various types of infraredlasers emitting infrared radiation in 9 to 12 micrometer wavelength,particularly 10.6 micrometers. The removal of elastomeric material bythe laser can be aided by the presence of the additive sensitive toinfrared radiation in the reinforced elastomeric layer which absorbs theradiation energy generated by the laser. A laser which is particularlysuitable for engraving reinforced elastomeric precursors for reliefprinting is a carbon dioxide laser which emits at 10.6 micrometerwavelength. Carbon dioxide lasers are commercially available at areasonable cost. The carbon dioxide laser can operate in continuous-waveand/or pulse mode. Capability of operating the laser in both modes isdesirable since at low or moderate radiation intensities, pulseengraving may be less efficient. Energy which might heat, even melt thematerial, but not vaporize it or otherwise cause it to become physicallydetached is lost. On the other hand, continuous wave irradiation at lowor moderate intensities is accumulated in a given area while the beamscans the vicinity of that area. Thus at low intensities continuous wavemode may be preferred. Pulsed mode may be the preferred mode at highintensities because if a cloud of radiation absorbing material wereformed, there would be time for it to dissipate in the time intervalbetween pulses and, thus, it would permit a more efficient delivery ofradiation to the solid surface.

The one laser-engravable element that is secured to the carrier can bemounted onto an exterior of a rotating drum associated with the laser.The laser is focused to impinge on the element on the drum. As the drumis rotated and translated relative to the laser beam, the element isexposed to the laser beam in a spiral fashion. The laser beam ismodulated with image data, resulting in a two dimensional image withrelief engraved into the element, that is, a three-dimensional element.Relief depth is the difference between the thickness of the floor andthe thickness of the printing layer. Alternately, the laser may moverelative to the element on the drum.

Because laser-engraving removes the elastomeric material in depth andforms the relief surface suitable for printing, the composite printingform does not undergo treating step, but can undergo post-exposureand/or finishing exposures.

After treatment or engraving, the thus prepared composite printing formis then ready for mounting onto a printing cylinder. The method formounting the composite printing form is not limited. One method formounting relief printing form onto a printing cylinder attaches amounting bar to the leading edge of the composite printing form viaremovable pins inserted in the leading edge holes and the mounting bar,as described by Fox et al. in U.S. Pat. No. 5,562,039.

Photosensitive Element

The photosensitive element used for preparing flexographic printingforms includes at least one layer of a photopolymerizable composition.The term “photosensitive” encompass any system in which the at least onephotosensitive layer is capable of initiating a reaction or reactions,particularly photochemical reactions, upon response to actinicradiation. In some embodiments, the photosensitive element includes asupport for the photopolymerizable layer. In some embodiments, thephotopolymerizable layer is an elastomeric layer that includes a binder,at least one monomer, and a photoinitiator. In some embodiments, thephotosensitive element includes a layer of an infrared sensitivematerial which can also function as an actinic radiation opaque materialadjacent the photopolymerizable layer, opposite the support.

Unless otherwise indicated, the term “photosensitive element”encompasses printing precursors capable of undergoing exposure toactinic radiation and treating to form a surface suitable for printing.Unless otherwise indicated, the “photosensitive element” and “printingform” includes elements or structures in any form which become suitablefor printing or are suitable for printing, including, but not limitedto, flat sheets, plates, plates-on-sleeves, and plates-on-carriers. Itis contemplated that printing form resulting from the photosensitiveelement has end-use printing applications for relief printing, such asflexographic and letterpress printing. Relief printing is a method ofprinting in which the printing form prints from an image area, where theimage area of the printing form is raised and the non-image area isdepressed.

The photosensitive element includes at least one layer of aphotopolymerizable composition. As used herein, the term“photopolymerizable” is intended to encompass systems that arephotopolymerizable, photocrosslinkable, or both. The photopolymerizablelayer is a solid elastomeric layer formed of the composition comprisinga binder, at least one monomer, and a photoinitiator. The photoinitiatorhas sensitivity to actinic radiation. Throughout this specificationactinic light will include ultraviolet radiation and/or visible light.The solid layer of the photopolymerizable composition is treated withone or more solutions and/or heat to form a relief suitable forflexographic printing. As used herein, the term “solid” refers to thephysical state of the layer which has a definite volume and shape andresists forces that tend to alter its volume or shape. The layer of thephotopolymerizable composition is solid at room temperature, which is atemperature between about 5° C. and about 30° C. In most embodiments, asolid layer of the photopolymerizable composition of the photosensitiveelement is unpolymerized, but can include some embodiments where thephotopolymerizable composition may be polymerized (photohardened), orboth polymerized (i.e., floor) and unpolymerized.

The binder is not limited and can be a single polymer or mixture ofpolymers. In some embodiments, the binder is an elastomeric binder. Inother embodiments, the binder becomes elastomeric upon exposure toactinic radiation. Binders include natural or synthetic polymers ofconjugated diolefin hydrocarbons, including polyisoprene,1,2-polybutadiene, 1,4-polybutadiene, butadiene/acrylonitrile, anddiene/styrene thermoplastic-elastomeric block copolymers. In someembodiments, the binder is an elastomeric block copolymer of an A-B-Atype block copolymer, where A represents a non-elastomeric block, and Brepresents an elastomeric block. The non-elastomeric block A can be avinyl polymer, such as for example, polystyrene. Examples of theelastomeric block B include polybutadiene and polyisoprene. In someembodiments, the elastomeric binders includepoly(styrene/isoprene/styrene) block copolymers andpoly(styrene/butadiene/styrene) block copolymers. The binder can besoluble, swellable, or dispersible in aqueous, semi-aqueous, water, ororganic solvent washout solutions. Generally, the elastomeric binderswhich are suitable for washout development are also suitable for use inthermal treating wherein the unpolymerized areas of thephotopolymerizable layer soften, melt, or flow upon heating.

Either a single elastomeric material or a combination of materials canbe used for the elastomeric layer so long as the characteristics desiredfor relief printing are obtained. Examples of elastomeric materials aredescribed in Plastic Technology Handbook, Chandler et al., Ed., (1987).In many cases it may be desirable to use thermoplastic elastomericmaterials to formulate the elastomeric layer. When a thermoplasticelastomeric layer is reinforced photochemically, the layer remainselastomeric but is no longer thermoplastic after such reinforcement.This includes, but is not limited to, elastomeric materials such ascopolymers of butadiene and styrene, copolymers of isoprene and styrene,styrene-diene-styrene triblock copolymers.

In addition to the elastomeric binder, a second binder may optionally bepresent in the elastomeric layer. A suitable second binder includesnon-crosslinked polybutadiene and polyisoprene; nitrile elastomers;polyisobutylene and other butyl elastomers; polyalkyleneoxides;polyphosphazenes; elastomeric polymers and copolymers of acrylates andmethacrylate; elastomeric polyurethanes and polyesters; elastomericpolymers and copolymers of olefins such as ethylene-propylene copolymersand non-crosslinked EPDM; elastomeric copolymers of vinyl acetate andits partially hydrogenated derivatives.

The photopolymerizable composition contains at least one compoundcapable of addition polymerization that is compatible with the binder tothe extent that a clear, non-cloudy photosensitive layer is produced.The at least one compound capable of addition polymerization may also bereferred to as a monomer. Monomers that can be used in thephotopolymerizable composition are well known in the art and include,but are not limited to, addition-polymerization ethylenicallyunsaturated compounds with at least one terminal ethylenic group. Thecomposition can contain a single monomer or a combination of monomers.Monomers can be appropriately selected by one skilled in the art toprovide suitable elastomeric and other properties to thephotopolymerizable composition.

The photoinitiator can be any single compound or combination ofcompounds which is sensitive to actinic radiation, generating freeradicals which initiate the polymerization of the monomer or monomerswithout excessive termination. Any of the known classes ofphotoinitiators, particularly free radical photoinitiators may be used.Alternatively, the photoinitiator may be a mixture of compounds in whichone of the compounds provides the free radicals when caused to do so bya sensitizer activated by radiation. Preferably, the photoinitiator forthe main exposure (as well as post-exposure and backflash) is sensitiveto visible or ultraviolet radiation, between 310 to 400 nm, andpreferably 345 to 365 nm.

The photopolymerizable composition can contain other additives dependingon the final properties desired. Additional additives to thephotopolymerizable composition include sensitizers, plasticizers,rheology modifiers, thermal polymerization inhibitors, colorants,processing aids, antioxidants, antiozonants, dyes, and fillers.

The thickness of the photopolymerizable layer can vary over a wide rangedepending upon the type of printing form desired. In some embodiments,the photosensitive layer can have a thickness from about 0.002 inch toabout 0.250 inch or greater (about 0.051 mm to about 0.64 cm orgreater). Typical thickness of the photopolymerizable layer is fromabout 0.010 inches to about 0.250 inches (about 0.025 cm to about 0.64cm). In some embodiments, the photosensitive layer can have a thicknessfrom about 0.107 inch to about 0.300 inch (about 0.27 to about 0.76 cm).

The photosensitive element typically includes a support adjacent thelayer of the photopolymerizable composition. The support can be composedof any material or combination of materials that is conventionally usedwith photosensitive elements used to prepare printing forms. In someembodiments, the support is transparent to actinic radiation toaccommodate “backflash” exposure through the support. Examples ofsuitable support materials include polymeric films such those formed byaddition polymers and linear condensation polymers, transparent foamsand fabrics, such as fiberglass. Under certain end-use conditions,metals such as aluminum, steel, and nickel, may also be used as asupport, even though a metal support is not transparent to radiation. Insome embodiments, the support is a polyester film. In one embodiment,the support is polyethylene terephthalate film. In some embodiments, thesupport has a thickness from 0.002 to 0.050 inch (0.0051 to 0.127 cm).In other embodiments, the thickness for the sheet form support is 0.003to 0.016 inch (0.0076 to 0.040 cm).

Optionally, the element includes an adhesive layer between the supportand the photopolymerizable layer, or a surface of the support that isadjacent the photopolymerizable layer has an adhesion promoting surface.Further, the adhesion of the photopolymerizable layer to the support canbe adjusted by exposing the element to actinic radiation through thesupport as disclosed by Feinberg et al. in U.S. Pat. No. 5,292,617.

As is well known to those of ordinary skill in the art, thephotosensitive element may include one or more additional layersadjacent the photopolymerizable layer, that is, on a side of thephotopolymerizable layer opposite the support. Depending on desired use,the additional layers may be opaque or transparent to actinic radiation,and may have one or more functions for the photosensitive element. Theadditional layers include, but are not limited to, a release layer, anelastomeric capping layer, a barrier layer, an adhesion modifying layer,a layer which alters the surface characteristics of the photosensitiveelement, and combinations thereof. The one or more additional layers canbe removable, in whole or in part, during treatment. One or more of theadditional layers may cover or only partially cover the photosensitivecomposition layer. It is well within the ordinary skill of those in theart to select and prepare additional layers on the photopolymerizablelayer according to desired end-use.

In one embodiment, the photosensitive element includes the infraredsensitive layer which is also an actinic radiation opaque layer disposedabove a surface of the photopolymerizable layer opposite the support.The actinic radiation opaque layer may substantially cover the surfaceor only cover an imageable portion of the photopolymerizable layer. Theactinic radiation opaque layer is substantially opaque to actinicradiation that corresponds with the sensitivity of thephotopolymerizable material. The actinic radiation opaque layer can beused with or without a barrier layer. If used with the barrier layer,the barrier layer is disposed between the photopolymerizable layer andthe radiation opaque layer. If present, the barrier layer can minimizemigration of materials between the photopolymerizable layer and theradiation opaque layer. The actinic radiation opaque layer is alsosensitive to laser radiation that can selectively remove or transfer theopaque layer. In one embodiment, the actinic radiation opaque layer issensitive to infrared laser radiation. The actinic radiation opaquelayer comprises a radiation-opaque material, an infrared-absorbingmaterial, and an optional binder. Dark inorganic pigments, such ascarbon black and graphite, mixtures of pigments, metals, and metalalloys generally function as both infrared-sensitive material andradiation-opaque material. The optional binder is a polymeric materialwhich is not limited. The thickness of the actinic radiation opaquelayer should be in a range to optimize both sensitivity and opacity,which is generally from about 20 Angstroms to about 50 micrometers. Theactinic radiation opaque layer should have a transmission opticaldensity of greater than 2.0 in order to effectively block actinicradiation and the polymerization of the underlying photopolymerizablelayer.

The photosensitive printing element of the present invention may furtherinclude a temporary coversheet on top of the uppermost layer of theelement. One purpose of the coversheet is to protect the uppermost layerof the photosensitive printing element during storage and handling.Examples of suitable materials for the coversheet include thin films ofpolystyrene, polyethylene, polypropylene, polycarbonate, fluoropolymers,polyamide or polyesters, which can be subbed with release layers. Thecoversheet is preferably prepared from polyester, such as Mylar®polyethylene terephthalate film; most preferably the coversheet is 5-milMylar®.

The photosensitive element on the composite printing form, afterexposure (and treating) of the photosensitive element, has a durometerof about 20 to about 80 Shore A. In some embodiments, the (exposed andtreated) photosensitive element on the composite printing form has adurometer of 30 to 50 Shore A. The Shore durometer is a measure of theresistance of a material toward indentation. Durometer of Shore A is thescale typically used for soft rubbers or elastomeric materials, wherethe higher the value the greater the resistance toward penetration. Thedurometer of the printing form can be measured according to standardizedprocedures described in DIN 53,505 or ASTM D2240-00.

1. A method for preparing a composite printing form comprising: a)providing one photosensitive element comprising a layer of aphotopolymerizable composition and an infrared sensitive layer disposedabove the photopolymerizable layer, the element having a planar area; b)positioning the element adjacent a carrier having no marking orindicator for positioning of the element, wherein the carrier has aplanar area, and the planar area of the one photosensitive element is atleast 70% of the planar area of the carrier; c) securing the element tothe carrier; d) imagewise exposing the infrared sensitive layer withinfrared laser radiation to form a mask on the element; e) overallexposing the element to actinic radiation through the mask; and f)treating the element of e) to form a relief structure on the carrier. 2.The method of claim 1 wherein the carrier has a leading edge, and thepositioning step further comprises locating a leading edge of the oneelement at a distance from the leading edge of the carrier.
 3. Themethod of claim 2 wherein the distance is sufficient to fit a mountingbar to the carrier.
 4. The method of claim 1 further comprisingattaching a mounting bar to the leading end of the carrier.
 5. Themethod of claim 1 wherein the element has a length and a width that isless than a length and a width of the carrier.
 6. The method of claim 1wherein the positioning step further comprises providing a margin ofopen space on the carrier surrounding the element.
 7. The method ofclaim 1 wherein 70% to about 99% of the planar area of the carrier iscovered by the one photosensitive element.
 8. The method of claim 1wherein the planar area of the one photosensitive element is 70% to 98%of the planar area of the carrier.
 9. The method of claim 1 wherein theplanar area of the one photosensitive element is 75% to 95% of theplanar area of the carrier.
 10. The method of claim 1 further comprisingpunching at least two registration holes in a leading edge of thecarrier.
 11. The method of claim 1 further comprising prior to step d),attaching a mounting bar to the leading end of the carrier, and securingthe mounting bar with the carrier on a drum in register by the imagewiseexposure.
 12. The method of claim 1 wherein the imagewise exposing stepd) comprises ablating the infrared sensitive layer from thephotopolymerizable layer.
 13. The method of claim 1 wherein after stepf), further comprising removing the element from the carrier, andre-using the carrier in preparing a second composite printing form witha second photosensitive element according to steps b) through f).
 14. Amethod for preparing a composite printing form comprising: a) providingone laser-engravable element comprising a reinforced elastomeric layer,the element having a planar area; b) positioning the element adjacent acarrier having no marking or indicator for positioning of the element,wherein the carrier has a planar area, and the planar area of the oneelement is at least 70% of the planar area of the carrier; c) securingthe element to the carrier; and d) exposing the reinforced layer withlaser radiation to selectively remove the reinforced layer in depth andform a relief structure on the carrier.
 15. The method of claim 14wherein the precursor is a photosensitive element and the elastomericlayer is reinforced by overall exposure to actinic radiation.